Different node_environment for provider/requestor nodes

[johny-b]

There are some variables, like YA_PAYMENT_NETWORK, that are currently set, but are used only by the provider agent.
This is confusing, we'd rather set only variables that are really used for a given node.

Currently there is no internal provider/requestor distinction, but it could be added easily (e.g. something like node_type.class.is_provider).